Wolfenstein: Enemy Territory was originally planned as an add-on for Return to Castle Wolfenstein, but soon became it's own game, a separate entity with full single player, multiplayer, AI opponents and teammates! Well that didn't happen and what we got, while less than planned, is amazing...and FREE! Wolfenstein: Enemy Territory is a free, stand-alone multiplayer only product, which means it's for online play only, but all you need is what you download. You do not need anything else to play Wolfenstein: Enemy Territory, than what you download for free. No Return to Castle Wolfenstein, no additional file packages or monthly charges, just one phat lump of action, and did I mention it was free? (Sorry but people still can't believe it).
Source: The Game Website.